In the manufacture of extrusion foam there are competing factors to balance. One needs to consider the viscosity or melt flow rate of the polymer as it impacts on the extruder output and the melt strength of the polymer, and particularly of the foaming mass as it leaves the extruder as this impacts on the bubble stability or the foam stability. If one makes a very low viscosity polymer it will flow through the extruder easily. However a low viscosity polymer tends to have a low melt strength and the resulting foam tends to have a lower stability. Accordingly, there is a tendency for foams of low viscosity to collapse upon extrusion or shortly after leaving the extruder.
It has been known for some time that the melt strength of a polymer may be improved by lightly cross linking the polymer. The paper "Some Effects of Crosslinking Upon the Foaming Behavior of Heat Plastified Polystyrene", L. C. Rubens Journal of Cellular Plastics, April 1965, 311-320 discloses that polystyrene, containing small amounts (about 0.03 weight %) of divinyl benzene, may be foamed with CO.sub.2 and the polymer has good foam stability and good foam volume. This technology is also the subject matter of U.S. Pat. Nos. 2,848,427 and 2,848,428 issued Aug. 19, 1958 to Louis C. Rubens assigned to The Dow Chemical Company. The technology comprised forming a cross linked polystyrene polymer then impregnating it in solid state with CO.sub.2 then releasing the pressure and letting the polymer expand. This technology was not strongly relevant to extrusion foam techniques.
The cross linking technology was further applied in U.S. Pat. No. 3,960,784 issued Jun. 1, 1976 to Louis C. Rubens assigned to The Dow Chemical Company. This patent teaches concurrent impregnation of a polymer with a blowing agent and a cross linking agent. The polystyrene is prepared at temperatures from about 60.degree. C. to 120.degree. C., preferably from about 70.degree. C. to 100.degree. C. (Column 3 lines 25-26). These temperature ranges are indicative of suspension polymerization and concurrent or post polymerization impregnation with the blowing agent and cross linking agent (see Example 3) although the polymer could be molded into thin sheets for the impregnation step. This reference does not teach an extrusion foam.
While divinyl benzene is useful in suspension polymerization it tends to produce gels in bulk or solution polymerization. In a bulk or solution polymerization the use of tetra functional initiators significantly reduces gels. Typically no or very low levels (e.g. less than 0.5 weight %, more generally less than 0.1 weight %) of gels (i.e. insoluble polymer in typical solvents).
With the Montreal protocol on reducing the use of CFC's and HCFC's and regulations regarding the permissible discharge of volatile organic compounds (VOC's) there was increase pressure on polymer foam industry to move to other blowing agents such as CO.sub.2 or N.sub.2. Representative of this type of art is Monsanto's Australian Patent 529339 allowed 1713/83. The patent teaches the formation of a foam by extruding polystyrene and injecting CO.sub.2 into the extruder. Interestingly there is no mention of cross linking agents or branched polystyrene in the patent. U.S. Pat. No. 5,250,577 issued Oct. 5, 1993 to Gary C. Welsh is similar as it pertains to extrusion foaming polystyrene in an extrusion process using CO.sub.2 as the sole blowing agent. Again there is no reference in U.S. Pat. No. 5,250,577 to the use of cross linking agents.
At about this time U.S. Pat. No. 5,266,602 issued to Walter et al. assigned to BASF. The patent teaches foaming a branched polystyrene. The foaming agent is conventional (e.g. C.sub.4-6 alkanes). The polymer is prepared in the presence of a peroxide initiator other than a benzoyl compound and a moderator (chain transfer agent) such as a mercaptan (e.g. t-dodecyl mercaptan) and a "branching agent". The branching agent contains a second unsaturation as a point for the polymer to branch. Suitable agents include divinyl benzene, butadiene and isoprene. These types of branching agents would not produce the star branched polymers referred to herein. The actual polymerization process appears to be a suspension process. Additionally there is no reference in the disclosure to blowing the polystyrene with anything other than conventional alkane blowing agents.
U.S. Pat. No. 5,576,094 issued Nov. 19, 1996 to Callens et al. assigned to BASF teaches extruding slab foamed polystyrene blown with CO.sub.2 or a mixture of CO.sub.2 and C.sub.1-6 alcohols or ethers of C.sub.1-4 alkyl alkoxy compounds. The polystyrene is a branched polystyrene preferably having at least 50%, more preferably 60% of the polymer being a star branched styrene butadiene block polymer. The polymer has a VICAT softening temperature not greater than 100.degree. C. This teaches against the subject matter of the present invention. Additionally the polymer has a melt index MVI 200/5 of at least 5 mL/10 minutes.
U.S. Pat. No. 5,830,924 issued Nov. 3, 1998 to Suh et al. assigned to The Dow Chemical Company claims a process for extruding a closed cell foam using CO.sub.2 or a mixture of CO.sub.2, conventional alkane blowing agents and a polystyrene in which from 50 to 100 weight % of the polystyrene is star branched (i.e. branched). This teaches away from the subject matter of the present invention which requires a different type of polymer and lower weight % of star branched vinyl aromatic polymer.
U.S. Pat. No. 5,760,149 issued Jun. 2, 1998 to Sanchez et al. discloses tetrafunctional (monoperoxycarbonate) compounds that are useful as initiators for olefin monomers including styrene. The patent also teaches a process for polymerizing polystyrene. However, there is no teaching in the patent of foaming the resulting polymer using extrusion techniques.
The present invention seeks to provide a novel process for extrusion foaming of styrenic polymers in which the styrenic polymer comprises less than 50 weight % of branched styrenic polymer.